This invention pertains to the measurement of radial profiles of energy components of intense particle beams, and relates especially to neutral beams used in the heating of a fusion plasma.
A number of approaches are currently being studied in the development of nuclear fusion as a long-term energy source. Several of the more promising approaches involve the confinement, by means of strong magnetic fields, of a highly energetic plasma possessing extremely high temperature and densities, so as to cause the fusing of atoms, such as deuterium and tritium, and the resulting production of energy.
It has been found that one of the most efficient configurations for optimum plasma containment is in the form of a toroid or "doughnut". This has given rise to the tokamak fusion reactor design which is currently under intensive study by research groups in a number of countries. By means of a circular arrangement of powerful magnets, a toroidal magnetic field is formed for the confinement of an energetic plasma comprised primarily of protons and deuterons.
Once generated, the confined energetic plasma must be sustained by means of an external source. Further, the plasma must be continuously re-fueled during reactor operation. One method of heating and fueling the plasma, and thus energizing the plasma particles confined, includes the injection of a beam of energetic neutrals into a plasma. Neutral beam injection methods have become technologically feasible with the recent development of efficient, high-power neutral beam modules such as those described in the following: V. D. Shafranov and E. I. Yurchenko, Nuclear Fusion 8, 329 (1968); L. D. Stewart, R. C. Davis, J. T. Hogan, T. C. Jernigan, O. B. Morgan, and W. L. Stirling, Garching/Munich Conference, 1973, paper E 12; W. L. Stirling, R. C. Davis, T. C. Jernigan, O. B. Morgan, J. J. Orzechowski, G. Schilling and L. D. Stewart, paper d-5 at Second International Conference on Ion Sources, Vienna, 1972; K. W. Ehlers and W. B. Kunkel, paper d-3 at Second International Conference on Ion Sources, Vienna, 1972; and W. L. Gardner et al, Rev. Sci. Inst. 53, 424 (1982). Since the beam power absorbed by the plasma exceeds the ohmic heating power, neutral beams cause significant heating. Neutral beams also provide a particle source to offset the fusion losses, and could be used to maintain an electric current in the plasma, as described in the following: T. Ohkawa, Nuclear Fusion 10, 185 (1970); R. J. Bickerton, Comments on Plasma Phys. and Controlled Fusion 1, 95 (1972); and J. D. Callen, J. F. Clarke and J. A. Rome, Garching/Munich Conference, 1973, paper E-14. The neutral beam injection method of plasma heating is currently being tested in a number of research facilities, including the Poloidal Divertor Experiment (PDX) located at the Princeton Plasma Physics Laboratory in New Jersey, the ISX-B project at the Oak Ridge National Laboratory, the Doublet III project at GA Technologies, and the ASDEX studies at the Institute for Plasma Physics in Garching, West Germany.
A typical neutral beam injector consists of only a few basic parts. First, there is a plasma source, such as the duo-PiGatron. Ions extracted from the plasma meniscus are accessed through a multiple-aperture (approximately 2000, 3-4 mm diameter hole) plate. The ions are then accelerated in a multiple aperture accel-decel electrode system to typical energies of 40-50 keV, and enter a charge-exchange neutralization cell which contains a neutral gas such as H.sub.2 or D.sub.2. At the neutralization cell outlet, about 60-80% of the ions have been converted into energetic neutrals, which are then injected into the fusion plasma. Ions remaining at the end of the charge-exchange cell are bent out of the beam path by an ion deflection magnet usually contained within the vacuum enclosure of the injection system. As the energetic neutrals from the neutral beam penetrate into the confined fusion plasma, they undergo ionization by charge-exchange with the plasma ions. After being "ionized," the fast ions from neutral beam injection circulate around the fusion reactor along particle orbits that follow the magnetic field lines. Plasma heating results from the slowing down of the fast ions by collisions with the background plasma ions and the electrons. For fusion plasma heating, neutral beam efficiencies are found to be a very sensitive function of neutral species energy yields of the neutral beam. Hence, practical neutral beam injection systems should include a provision for accurate determination of energy species yields.
High energy particle beams have also found wide use in science and industry. In typical applications, beams are first accelerated to a desired energy, and are then used to analyze or modify various targets. In nuclear research for example, particle beams are used to analyze the basic physical properties of subatomic systems, whereas in solid state physics research, particle beams are used to study the physical properties of surfaces, crystals, and thin films. Beams are also used to modify targets, as in the commercial production of radio-pharmaceuticals, ion-implanted semiconductor devices, and the heating of fusion reactor plasmas. Most particle beam applications require a control or knowledge of beam properties, such as energy, mass, charge, and species (i.e., energy components). Typical acceleration methods usually produce particle beams containing several different simultaneously-appearing atomic species. This is due to the variety of atomic processes that occur in the ion sources from which particle beams are extracted prior to acceleration. For example, a deuterium ion source is fed diatomic deuterium gas (D.sub.2) from a gas cylinder. The deuterium gas, upon entering the ion source, is decomposed into ions of atomic deuterium (D+), ions of diatomic molecular deuterium (D.sub.2.sup.+), and stable ions of triatomic molecular deuterium (D.sub.3.sup.+). The extraction and acceleration of these ions to an energy E produces a beam with three molecular species, each of which initially have the same energy. However, collisions of the molecular D.sub.2.sup.+ and D.sub.3.sup.+ beam species with gas in the beam acceleration system causes a decomposition of these molecular species into atomic particles having energies of one-half (E/2) and one-third (E/3) of the initial acceleration energy (E). Thus, the resulting beam can consist of particles having several different energies.
Similar processes occur with other kinds of beams. In some applications, it is necessary to filter the ion beam using magnetic or electrostatic devices to prevent undesirable beam species from reaching the target. In other applications, such as the beam injecting systems used to heat the fusion reactor plasmas for example, pre-neutralization beam filtering is impractical due to the size, cost, and complexities involved. It is nevertheless desirable to know the neutral energy species content of such beams in order to optimize ion source performance in the beam-target interaction. Even in cases where beam filtering is used, it may be desirable to have the capability of detecting the presence of unwanted energy species resulting from system inefficiencies or defects.
Several well-known methods of measuring the energy species content of particle beams are surveyed in an article by C. C. Tsai et al, Oak Ridge National Laboratory Technical Memo, ORNL TM-8360, Aug. 8, 1982. Each of these methods has limitations. For example, energy and momentum analysis methods which use electric/magnetic fields are limited to relatively low beam-currents, usually at a single point in the beam, and require that the beam particles be in a known ionic charge state. If the beam consists of neural particles, a gas-stripper cell or foil must be used to ionize the beam prior to analysis. Since the analysis still requires electric/magnetic fields, the same limitations just mentioned apply, with added complexity stemming from the gas handling and support requirements of gas cells or foils.
Optical diagnostic methods of detecting the Doppler shift in the wavelength of light emitted by beam species moving at different velocities require relatively high background gas pressures to give detectable light output. The beam line regions having sufficient gas pressure for optical diagnostics are frequently located far from the target region, thereby allowing undetected changes to occur in the beam before the beam strikes the target. In the case of powerful neutral beams used to heat fusion reactor plasmas, optical species measurements must be made near the ion source, in a region containing both neutral and ionic particles which thereafter pass through a high gas density. Hence, measurements made with these diagnostic techniques must be extrapolated in order to estimate the state of the beam as it interacts with the target plasma.
It is therefore an object of the present invention to provide particle beam energy species analyses applicable to either ion or neutral beams, that allows accurate, prompt, direct, position-dependent, in-situ measurements of the beam energy species.
Operating tokamaks constitute a hostile environment for any in situ diagnostic technique, and concern over stray magnetic fields and electrical noise have discouraged proposals to consider sensitive particle in-situ measurements.
It is an object of the present invention to provide analyses of the aforementioned type for fusion reactor plasmas which are heated by intense neutral beams, where heating efficiency is a sensitive function of D.degree.(E), the full energy species component of the heating beam.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.